The present invention relates generally to circuit boards. More particularly, the present invention relates to circuit board layout.
Modern integrated circuits have attained a complexity that requires a large number of terminals for connection to other devices. To reduce the package size of such integrated circuits, the terminals are generally arranged in multiple rows on the underside of the package, for example in what is referred to as a “ball grid array” (BGA). These terminals, referred to as “balls,” are then connected to contacts called “lands” on the surface of the circuit boards. Connections called “traces” connect the lands to other devices or terminals on the circuit board.
To reduce package footprint, the balls on the integrated circuit package are generally placed as close together as possible, as are the lands on the circuit board.
FIG. 1 shows a top circuit board layer 100 for a prior art circuit board comprising three rows 108A-C of lands 102. As can be seen in FIG. 1, lands 102 are generally so close together that only one trace 104 can pass between adjacent lands 102. In fact, as can be seen in the arrangement of FIG. 1, there is no room for traces 104 to be connected to the lands 102 in the third row 108C. Instead, a second circuit board layer is required. The lands 102 in the third (and higher) rows must be connected to vias, which penetrate circuit board layer 100 to connect to traces on other circuit board layers.
A similar limitation occurs for internal layers of prior art circuit boards. FIG. 2 shows an internal circuit board layer 200 of a prior art circuit board comprising three rows 208A-C of vias 206. As can be seen in FIG. 2, vias 206 are also generally so close together that only one trace 104 can pass between adjacent vias 206. In fact, as can be seen in the arrangement of FIG. 2, there is no room for traces 104 to be connected to the vias 206 in the third row 208C. Instead, a further circuit board layer is required. The vias 206 in the third (and higher) rows penetrate circuit board layer 200 to connect to traces on other internal circuit board layers.
As can be seen in FIGS. 1 and 2, a device having many rows of balls requires several circuit board layers. In addition, power and ground plane layers are required for each circuit board layer. These many layers increase the complexity and cost of the circuit boards.